1. Field of the Invention
The present invention relates to an optical element having an entrance refractive surface, an exit refractive surface and internal reflective surfaces, a holding structure for the optical element, and an image pickup apparatus.
2. Description of Related Art
Image pickup in electronic still cameras or video cameras is performed by converting an object image formed on an image pickup element (hereinafter represented as a CCD, by way of an example) by an optical system into electrical signals by means of the CCD.
A range within which a sufficiently sharp image for actual usage is available before and behind the best imaging point by the optical system is called the depth of focus. To perform image pickup without defocusing or partial-defocusing, it is necessary to position a plane of the CCD within the depth of focus. The optical system and image pickup system having the autofocus mechanism conduct adjustment of the position of a lens group by detecting the focus in order to position the CCD plane always within the depth of focus. On the other hand, the optical system and image pickup system having no autofocus mechanism require the back-focus adjustment for adjusting the distance of the CCD plane with respect to the optical system.
The back-focus adjustment is performed, for example, in the adjustment step during the assembly process by using jigs and tools, or after the assembly process by integrating an adjustment mechanism within the holding member for the CCD.
The former of the above-described back-focus adjustment has such disadvantages that replacement or recycling of the CCD is not possible because the CCD is completely fixed by adhesives after adjustment, and the adjustment step takes much time.
Now, an example of the latter of the above-described back-focus adjustment is shown in FIGS. 9A and 9B. FIG. 9A is a sectional view showing a cross-section of a typical single-focus lens unit and image pickup system along a plane including the optical axis. In FIG. 9A, the CCD is not represented by a cross-section.
FIG. 9B is a plan view of the back-focus adjustment mechanism for the CCD, as viewed in the direction of arrows on a plane represented by the two-dotted chain line in FIG. 9A. In FIGS. 9A and 9B, reference numeral 101 denotes a lens group composed of a plurality of lens elements, reference numeral 102 denotes a lens barrel for holding the lens group 101 and blocking light, reference numeral 103 denotes optical correction plates having a quartz low-pass filter and an infrared-cutting filter, reference numeral 104 denotes a CCD, reference numeral 104a denotes an image pickup plane of the CCD 104, reference numeral 104b denotes a corner portion of the image pickup plane 104a, reference numeral 105 denotes a CCD holding plate made of metal for holding the CCD 104, reference numeral 106 denotes a CCD circuit board for transmitting image signals to a flexible cable 108, etc., by electrically connecting terminals of the CCD 104 with soldering, etc, reference numeral 107 denotes an insulating member for insulating the CCD holding plate 105 from the CCD circuit board 106, reference numeral 109 denotes a base plate for holding the CCD holding plate 105 so as to allow adjustment of back-focus and tilt with respect to the lens barrel 102, reference numeral 109a denotes positioning holes for regulating the position of the base plate 109 with respect to the lens barrel 102, and reference numeral 109b denotes tapped holes for fastening the base plate 109 to the lens barrel 102 by screws. Reference numeral 110 denotes a back-focus feed screw for back-focus adjustment, reference numerals 111 and 112 denote a spring and an urging screw, respectively, for urging the CCD holding plate 105 toward the base plate 109, and reference numeral 113 denotes tilt adjusting screws for adjusting tilts around the x axis and y axis of the CCD holding plate 105.
The back-focus feed screw 110 engages with a tapped hole of the base plate 109 with the tip of the back-focus feed screw 110 in contact with the CCD holding plate 105. In an example shown in FIG. 9A, the tip of the back-focus feed screw 110 is arranged so as to come into contact with the rear side of the CCD holding plate 105 at a point corresponding to the corner portion 104b of the image pickup plane 104a of the CCD 104. The CCD 104 is fixed to the CCD holding plate 105 by bonding, etc., after positioning by a jig. The CCD holding plate 105 is moved by the back-focus feed screw 110 in directions for changing the distance from the optical system. More specifically, turning clockwise the back-focus feed screw 110 causes the CCD holding plate 105 to move toward the lens group 101, thereby decreasing the back-focus. Conversely, turning counterclockwise the back-focus feed screw 110 causes the CCD holding plate 105 to move away from the lens group 101, thereby increasing the back-focus.
Furthermore, the tilt of the CCD holding plate 105 is adjusted by the urging screw 112 and the two tilt adjusting screws 113. The urging screw 112 engages with a tapped hole of the CCD holding plate 105, and can freely move with respect to the base plate 109 along the axial direction thereof with positional restriction only in the direction of the plane of the base plate 109. The spring 111 is held in a compressed state between the urging screw 112 and the base plate 109 with the head of the urging screw 112 utilized, so as to apply an urging force to move the urging screw 112 in the direction away from the lens group 101 by a force of repulsion of the spring 111. This urging force causes a rotational moment working on the CCD holding plate 105 with the tip of the back-focus feed screw 110 acting as a fulcrum, but the tilt of the CCD holding plate 105 is restricted by two tilt adjusting screws 113. More specifically, the tilt adjusting screws 113 engage with tapped holes of the CCD holding plate 105 but can freely move with respect to the base plate 109, so that the tilt adjusting screws 113 are pulled in by the moment working on the CCD holding plate 105 to regulate the position of the CCD holding plate 105 at the location where the heads of the tilt adjusting screws 113 come into contact with the base plate 109. At this time, if an axis parallel to the longer side of the CCD image pickup plane 104a is set as the x-axis and an axis parallel to the shorter side as the y-axis out of axes passing through the corner portion 104b of the CCD image pickup plene 104a, the tilt adjusting screws 113 are disposed on the x-axis and the y-axis, respectively, to rotate the CCD holding plate 105 around the x-axis or the y-axis by turning either one of the tilt adjusting screws 113. Arranging the back-focus feed screw 110 and the tilt adjusting screws 113 in the above-described way allows performing adjustment readily in the directions around the x-axis or the y-axis independently.
Conducting the above operation while monitoring image signals from the CCD 104 generated by picking up an image of an evaluation chart allows performing adjustment of back-focus and tilt of the CCD 104. However, in the above-described back-focus adjustment method, the fulcrum of the CCD holding plate 105 shifts its position when the back-focus adjustment is performed. Therefore, the tilt adjustment is required always after the back-focus adjustment, so that it is impossible to perform the back-focus adjustment independently of the tilt adjustment.
Also, as shown in FIG. 9A, the lens group 101 is positioned in the direction perpendicular to the optical axis by the inner periphery of the lens barrel using the outer peripheries of the lens group 101 and in the direction along the optical axis by bringing the refractive surfaces of the lens group 101 into contact with the steps formed inside the lens barrel, and is then fastened by bonding or by a set ring or the like to the lens barrel. It is important for such an optical system to precisely align the positions of a plurality of lens elements and the positions of the lens group and the CCD. This is realized by enhancing the precision of the individual lens element and that of the lens barrel.
Typically, the conventional optical unit is constructed by holding a plurality of single lens in a lens barrel, and the optical unit is fixed to a camera body. The lens barrel is composed of a plurality of parts with complex shape, which require high precision assembly. Also, surface treatment such as painting or anodized aluminum is required for the parts of the lens barrel, resulting in expensive parts. Moreover, the size of the whole optical unit becomes considerably larger than the single lens.
Recently, further reduction in size of apparatuses is demanded due to increasing popularity of video cameras or electronic still cameras. Also, a demand for mobile communication terminals having the image information handling capability is rising as the information infrastructure has been developing. Further, research and development are being performed to reduce the size of an optical system, too, which is a constituent component of the apparatus for realizing the above demands. For example, as disclosed in U.S. Pat. No. 5,825,560 or U.S. patent application Ser. No. 08/606,825 filed Feb. 26, 1996, an optical element is proposed which is composed of two refractive surfaces and a plurality of reflective surfaces integrally formed on the surfaces of a transparent body so as to allow a light flux to enter from one of the refractive surfaces into the inside of the transparent body and, then, exit out of the other refractive surface after repeating reflections on the plurality of reflective surfaces.
This optical element enables the size thereof to be reduced compared to the conventional lens because it is possible to keep the light-passing effective aperture of each of the surfaces small by adopting a construction of transmitting an object image while repeating many times of imaging. Also, since any conventional lens barrel is not required, it is possible to further reduce the size, the number of parts and costs of the optical element.
The optical element must be positioned highly precisely in the image pickup apparatus. This is realized in the conventional lens by enhancing the precision of individual lens element and the lens barrel, as mentioned in the foregoing. However, a method for positioning of an optical element different from the conventional one is required for the present optical element, since one of its advantages lies in the capability of realizing the reduction of size of the apparatus by eliminating any lens barrel.
However, when such an optical element as having a plurality of integrally-formed reflective surfaces is used in constructing an image pickup apparatus, a problem arises in that the usage of the above-described conventional back-focus adjustment mechanism will hinder the reduction of size and cost of the apparatus. Since this problem reduces the value of the advantage of characteristics of the present optical element or capability of reducing size and cost, it is necessary to develop a back-focus adjustment method suitable for such an optical element.
Also, when such an optical element as having a plurality of integrally-formed reflective surfaces is used in constructing an image pickup apparatus, another problem arises in that the usage of the conventional construction of holding the optical element by a holding member as in the case of the conventional lens barrel for mounting the holding member to a camera body will increase the size of the apparatus and the number of parts and costs. Also, a further problem arises in that the usage of the construction of the conventional optical system covering the whole optical system with the lens barrel for the purposes of light-blocking and dust-proofing similarly results in an increase of size and cost of the apparatus.
In constructing an image pickup apparatus using an optical element, optical diaphragm means is required for regulating the amount of light entering the optical element. As for the optical diaphragm means, generally is used the so-called turret system in which a diaphragm blade having holes of various diameters is rotated manually or by a driving means, or the so-called IG-meter system in which an aperture is formed by a gap made by two or more diaphragm blades and the size of the aperture is varied by moving the diaphragm blades by means of a driving means. When such a diaphragm means is used in the present optical element, if the diaphragm blades are held by a holding member and the holding member including the diaphragm blades is disposed on the entrance surface of the optical element, a problem arises in that the advantage of the present optical system of realizing thinner profile is disrupted due to an increase in thickness of the image pickup apparatus by the thickness of the holding member.
As mentioned above, suitable methods of holding such an optical element and of disposing an optical diaphragm means are required.
In accordance with an aspect of the invention, there is provided an optical element, comprising an entrance refractive surface, an exit refractive surface and a plurality of internal reflective surfaces provided between the entrance refractive surface and the exit refractive surface, wherein a structure is provided for enabling the optical element to turn around a reference axis of the entrance refractive surface.
In accordance with another aspect of the invention, there is provided an image pickup apparatus, comprising an optical element having an entrance refractive surface, an exit refractive surface and a plurality of internal reflective surfaces provided between the entrance refractive surface and the exit refractive surface, and an diaphragm member which is disposed adjacent to the entrance refractive surface and is arranged to vary an aperture diameter thereof according to a movement thereof.
The above and further aspects and features of the invention will become apparent from the following detailed description of a preferred embodiment thereof taken in conjunction with the accompanying drawings.